1. The Invention
This invention relates to solid phosphoric acid catalyzed processes for polymerizing olefins (e.g. propylene). In a further aspect, the invention relates to a method of controlling the phosphoric acid strength of the catalyst in such processes to improve yields and catalyst life and reduce catalyst coking.
2. The Prior Art
The polymerization of olefins by contacting a mixed liquid/vapor stream of olefins with a solid phosphoric acid catalyst under polymerization conditions is well known and is, for example, disclosed in U.S. Pat. Nos. 2,596,497 and 2,909,580. The use of solid phosphoric acid catalysts was disclosed as early as 1935 in U.S. Pat. No. 1,993,513.
A major problem associated with this process is that water is removed from the solid catalyst as the process proceeds thereby increasing the phosphoric acid strength of the catalyst. This imposes a limitation on the useful life of the catalyst because the activity and selectivity of the catalyst is dependent upon exact control of the phosphoric acid strength. As the phosphoric acid strength exceeds about 104%, the formation of coke or tar is catalyzed thus substantially reducing yields and eventually requiring replacement of the catalyst as it becomes "coked-up".
This problem is recognized by the art and various solutions have been proposed. Two related solutions have been to supply water vapor along with the liquid olefin feed stream and/or to increase the amount of water in the catalyst. Both of these solutions have limitations which prevent them from replenishing any more than a portion of the water removed in the process. First, because of vapor-liquid equilibrium, insufficient water can be supplied in the vapor phase to replace the water removed during the reaction process. Moreover, water cannot be added in the liquid phase because it will severely weaken the catalyst support causing deterioration of at least a portion of the support into fine particles which in turn cause increased pressure drops through the bed and prematurely force catalyst change-out. Increasing the water in the catalyst is self-limiting and further limited by the fact the solid catalyst will deteriorate if too much water is used. It should be noted here that solid phosphoric acid catalysts are supported on Kieselguhr, or other similar materials which are very much less structurally stable than the impregnated or promoted refractory base catalysts used in other types of catalysts. Refractory base supports are generally quite stable to moisture, whereas the solid phosphoric acid catalyst supports soften and crumble into a sludge when excess moisture enters reaction. This requires premature process shutdown and increased maintenance.
A further approach described in U.S. Pat. No. 2,909,580 is the use of a solid thin film phosphoric acid catalyst with intermediate water washing steps. This approach is described as increasing the overall life of the catalyst but requires process shutdowns to perform the required water washing treatments.